The Washington Post has spent the past year determining how many children have been exposed to gun violence during school hours since the Columbine High massacre in 1999.

Beyond the dead and wounded, children who witness the violence or cower behind locked doors to hide from it can be profoundly traumatized.

The federal government does not track school shootings, so The Post pieced together its numbers from news articles, open-source databases, law enforcement reports and calls to schools and police departments.

Since March, The Post has taken a closer look at states with fewer local news sources and searched more deeply for less visible public suicides and accidents that led to injury.

The count now stands at more than 228,000 children at 234 schools.

The Post has found that at least 144 children, educators and other people have been killed in assaults, and another 302 have been injured.

In 2018 alone, there have already been 25 shootings — the highest number during any year since at least 1999. Still, school shootings remain rare, and only a tiny percentage of the tens of millions of students in America ever experience them.

The Post’s search for more shootings will continue, and it’s possible reporters will locate additional incidents from previous years.

Hundreds of outlets cover the deadliest attacks, such as the Feb. 14 rampage at Marjory Stoneman Douglas High in Parkland, Fla., where a 19-year-old man with an AR-15 rifle killed 17 people.

Others are covered by a single newspaper, such as a 2001 shooting at Pearl C. Anderson Middle School in Dallas, where a 14-year-old boy held a revolver to a girl’s chest and asked her whether she was “ready to die” before a bullet fired, grazing her hand.

Even as the list of incidents has expanded, however, the trend lines have remained consistent.

Among The Post’s most important findings: the disproportionate impact of school shootings on children of color.

Black students make up 16.6% of the school population...

...but they experience school shootings at twice that rate.

...but they experience school shootings at twice that rate.

In cases where the source of the gun could be determined, more than 85 percent of shooters brought them from their own homes or obtained them from friends or relatives, according to The Post’s analysis.

The ranks of school shooters include a 6-year-old boy, who killed a classmate after saying he didn’t like her, and a 15-year-old girl, who did the same to a friend for rejecting her romantic overtures.

Seven in 10 of them, however, were under the age of 18, which means that — often because of an adult’s negligence — dozens of children had access to deadly weapons.

The Washington Post spent a year determining how many children have been affected by school shootings, beyond just those killed or injured. To do that, reporters attempted to identify every act of gunfire at a primary or secondary school during school hours since the Columbine High massacre on April 20, 1999. Using Nexis, news articles, open-source databases, law enforcement reports, information from school websites and calls to schools and police departments, The Post reviewed more than 1,000 alleged incidents but counted only those that happened on campuses immediately before, during or just after classes.

Shootings at after-hours events, accidental discharges that caused no injuries to anyone other than the person handling the gun, and suicides that occurred privately or posed no threat to other children were excluded. Gunfire at colleges and universities, which affects young adults rather than kids, also was not counted.

After finding more than 200 incidents of gun violence that met The Post’s criteria, reporters organized them in a database for analysis. Because the federal government does not track school shootings, it’s possible that the database does not contain every incident that would qualify.

To calculate how many children were exposed to gunfire in each school shooting, The Post relied on enrollment figures and demographic information from the U.S. Education Department, including the Common Core of Data and the Private School Universe Survey. The analysis used attendance figures from the year of the shooting for the vast majority of the schools. Then The Post deducted 7 percent from the enrollment total because that is, on average, how many students miss school each day, according to the National Center for Education Statistics. Reporters subtracted 50 percent from a school’s enrollment if the act of gun violence occurred just before or after the school day. To provide information about school shootings since Columbine that fit The Post’s definition, send us an email at schoolshootings@washpost.com.

After mass shootings, much debate centers on Americans’ relatively easy access to guns. As of 2015, there are more guns than people in the United States, a rate that’s far higher than in other developed nations.